The trend in the production of integrated circuits is towards increasingly small structures on the chips. To produce chips of this type, therefore, it is necessary to employ lithography systems which are able to operate with ever smaller exposure wavelengths. Wavelengths of 248, 193 and 157 nm are currently used or under development. The use of electromagnetic radiation in what is known as the extreme UV region, in particular in the region from 11 to 14 nm, has been proposed for such lithography systems of the future. In this region, it will be necessary to change from a transmission system which has hitherto been customary to a reflection system with reflective optical elements and masks. Hitherto, only a small number of substrates for masks and also mirrors for the associated optics which are able to satisfy the extreme demands imposed on this technology have been proposed for reflective systems of this type.
DE 198 30 449 A1 describes a mirror substrate in which an amorphous thin film of quartz glass, amorphous silicon dioxide or aluminium oxide is applied to a substrate body with a high thermal conductivity.
U.S. Pat. No. 6,159,643 describes a reflection mask, comprising a layer which forms a pattern and a substrate, the substrate having a surface layer with an optical planarity of a quarter of a wavelength or better and a lower layer with a coefficient of thermal expansion α (or CTE) of less than 1.0 ppm/° C. in particular silicon is used as covering layer.
These two documents describe the use of silicon as one of the layers in a substrate for EUV lithography. However, silicon has a high thermal conductivity and a high thermal expansion, which is disadvantageous for EUV lithography.
WO 01/07967 and WO 01/08163 A1 describe masks and mirrors for EUV lithography in which titanium-doped high-purity silica glass is used as substrate. However, on account of their specific production process, these materials are generally insufficiently homogeneous for use in EUV lithography.
On account of their extremely low thermal expansion and very good homogeneity, glass-ceramics, such as Zerodur® are suitable for use as masks and/or mirrors for EUV lithography.
For this reason, initial optical systems based on Zerodur® substrate have been produced. It has thereby been established that, although these surfaces can be polished to an in-spec surface roughness of 0.1 nm rms using conventional methods, this roughness is lost during the ion beam etch which is subsequently customary. The surface roughness is increased by a factor of 2 to 5 following this treatment. However, since the surface roughness of 0.1 nm rms are a requirement for substrates for EUV lithography, the use of substrates of this type for optical systems which are suitable for production purposes is restricted.